Two injured as horrified onlookers say outcome could have been worse if the bus had not been stopped.

A runaway bus smashed into three other buses and a car on one of Glasgow's busiest streets after the driver forgot to apply the handbrake before getting out to buy a drink. The double decker vehicle reportedly rolled 100 metres downhill and through a set of traffic lights before eventually coming to a halt.

"Police are appealing for information following an incident involving a bus in Glasgow city centre on Monday 18 July 2016," a spokeswoman for Police Scotland said, according to the BBC.

"Around 15:45 police received a report of a bus apparently travelling south on Renfield Street having collided with a stationary car at traffic lights at West George Street. The car moved forward and struck a pedestrian on Renfield Street.

"The bus continued to travel on Renfield Street before it collided with a stationary bus near to St Vincent Street, which then nudged into another stationary bus on Renfield Street," she added. "Two men have been taken to the Glasgow Royal Infirmary for treatment to injuries.

"Inquiries are ongoing to establish the exact circumstances of the incident, including reviewing CCTV footage, and anyone with information is asked to contact Police Scotland on 101." None of the injuries are believed to be life threatening.

Describing the moment the horror dawned on the bus driver, Ali Hafiz, who was at the till at RF News said: "He was in my shop for a good two minutes before he realised he'd left the handbrake off.

"He had already been in Greggs, like a lot of bus drivers do when they stop here," he added. "He rushed out as soon as he realised his bus was moving slowly down the hill but still crashed into the Stagecoach bus."

According to eyewitnesses, the driverless double decker could have caused a major disaster had it not been stopped in its tracks, as it would have entered a busy Glasgow intersection.

"I saw the bus going down the street, the driver was chasing after it," one witness who did not wish to be named was quoted as saying by the Telegraph. "The bang was unbelievable. There was nobody on the bus, it hit a car and the driver was lying on the ground for about half an hour.

"It hit three other buses and stopped. God knows how or why this happened."